This invention relates to apparatus for measuring tension in a moving web, especially a web of a paper.
In the manufacture of paper products, web tension control is important to the production of products of consistent quality. Many different types of sensing and control devices have been used for this purpose, including dancer rolls controlling web speed, braking devices controlled by means for continuously measuring the braking force applied to the web, and load sensing devices in a variety of positions. Typical examples of prior art practice include many arrangements of draw rolls or drag bars partially wrapped by the web and provided with load sensing means positioned to sense either the tendency of the roll or bar to be displaced by web tension or the tendency of a non-rotating roll or bar to be turned by its frictional engagement with the moving web.
Such installations are subject to the disadvantage that while friction between the web and the roll is a significant factor, it varies considerably with the speed of the web, especially in the case of a non-rotating roll, because of the tendency of air to be entrained between the web and the roll as speed increases and thereby to serve as a lubricant decreasing the frictional effects. Experience has shown that the friction effect is greatest at low web speeds, that it decreases relatively gradually as web speeds increase, and that it effectively disappears suddenly at a speed of the order of 3,000 per minute, although this value is obviously subject to variation depending upon the frictional characteristics of the web and of the draw roll or bar which it wraps.
The practical result of these varying conditions as applied to the control of web tension is that measurement by means such as a load cell or strain gauge of the load on the draw load or bar is practicable at low web speeds even though the measured load includes friction as well as true web tension. A serious problem arises, however, in the speed range where the friction effect disappears, because this causes a correspondingly sudden change in the load sensed by the load cell or strain gauge, causing it to attempt to compensate for a change in web tension which did not actually take place, and thereby disrupting the actual tension conditions, sometimes to the point of breaking the web.
It is especially important to control web tension accurately in a web slitting line, by reason of the relatively high web speeds, which may range as high as 10,000 feet per minute. A station in a slitting line which is especially well adapted for cooperation with load sensing means is the drag roll or drag table assembly commonly used to support the moving paper web on each side of the slitter rolls. A typical such assembly incorporates a pair of rolls or a pair of arcuately curved shoes positioned on either side of the slitter rolls in such relation with a pair of guide rolls respectively upstream and downstream of the slitter rolls as to deflect the web out of the direct path between the guide rolls, and many arrangements of load sensing means have been combined with drag board assemblies.
One example of a drag board assembly incorporating load sensing means for continuously measuring tension in a moving paper web is shown in Canadian Pat. No. 931,985, wherein a load cell is positioned under one of the drag bars to measure the downward force applied thereto by the web. The arrangement in that patent, however, is such that the load cell responds to both web tension and the friction between the web and the drag board, and its accuracy is therefore affected by variations in web speed, especially through the value where the friction load between the web and the drag bar disappears.